1. Field of the Invention
The present invention relates to a raisable-lowerable member designed to be provided in a construction machine.
2. Background Art
Heretofore, there has been known a raisable-lowerable member designed to be provided in a construction machine. One example of such a raisable-lowerable member is disclosed, for example, in JP 60-061496A. A raisable-lowerable member disclosed in this patent publication comprises a boom, and a jib attached to the boom in such a manner as to be raisable and lowerable with respect to the boom. The jib disclosed in the patent publication is an enfolding support-type jib. As an axis of rotation of the enfolding support-type jib with respect to the boom, the jib has two rotational axes. A first one of the two rotational axes is a rotational axis during a crane work (see the reference code “F-1, B-1” in FIG. 5 of the patent publication). The first rotational axis corresponds to a rotational axis of a jib foot. The other, second, rotational axis is a rotational axis during a crane disassembling/reassembling operation (see the reference code “F-2, B-2” in FIG. 6 of the patent publication). The second rotational axis is a rotational axis of a support portion of the jib to be supported by the boom when the jib is set in an enfolded posture. The jib has two rotational axes in this manner, so that it becomes possible to suppress a bending moment which would otherwise be produced in the boom during a crane work, and allow the jib to be set in the enfolded posture (in a posture where the jib is disposed under the boom which has been lowered and laid down) during a crane disassembling/reassembling operation. Furthermore, based on the capability to allow the jib to be set in the enfolded posture, a space necessary for a crane disassembling/reassembling operation is minified.
The enfolding support type jib is equipped with an enfolding link mechanism (enfolding articulated mechanism) for coupling the jib and the boom together (see the reference codes 47, 49 in FIGS. 5 to 7 of the patent publication). The enfolding link mechanism comprises a first link (see the reference code 49 in FIG. 5 of the patent publication) rotatable about an axis coincident with a rotational axis of the jib foot, and a second link (see the reference code 47 in FIGS. 6 and 7 of the patent publication) rotatable about an axis coincident with a rotational axis of the support portion.
In the above conventional raisable-lowerable member, in order to attach the second link to the boom, it is necessary to positionally align a central axis of a distal end of the second link with a central axis of the jib foot (the details will be described later). That is, in the conventional raisable-lowerable member, during this operation, the distal end of the second link has to be moved by using an auxiliary crane or the like. For example, the distal end of the second link has to be moved up and down by using an auxiliary crane or the like. This gives rise to a problem that an attaching operation of the second link to the boom takes a lot of time and effort.